1. Field of the Invention
The present invention relates to a mask read only memory containing diodes, and in particular to a mask read only memory in which the logic “0” or “1” is defined by whether there is a dielectric layer on the diode.
2. Description of the Related Art
Nonvolatile memories are classified into mask read only memories (Mask ROM) and field programmable memories. In a conventional Mask ROM device, memory units are formed from metal-oxide-semiconductor transistors and the memory is programmed using ion implantation to regulate the threshold voltage of part of the MOS transistors, and thereby code logic into “0” and “1”.
In recent years, there has been a mask ROM using vertical PN diodes in memory cells. For example, U.S. Pat. No. 5,441,907 discloses a Mask ROM containing PN vertical diodes, wherein logic “0” and “1” are defined by whether there is a PN diode.
U.S. Pat. No. 6,185,122 B1 discloses a ROM containing PN diodes, which is a one time programmable read only memory and not a mask ROM. After the memory is manufactured, there is an oxide layer on the PN diode. The memory stores data of logic “0” and “1” by means of breaking down the oxide layer using high voltage energy, wherein the location that is broken down is defined as logic “1” and the location that is not broken down is defined as logic “0”. However, when the thickness of the oxide layer is not uniform, the definition is hindered.